1. Field of the Invention
The present invention relates generally to reflective type liquid crystal displays used as an information display board in information instruments, electric home appliances or the like, and more particularly, to a reflective type liquid crystal display capable of bright display without the absorption of light by a polarizer.
2. Description of the Background Art
Liquid crystal itself does not emit light but functions as a display by controlling incoming light transmitted therethrough. Methods of allowing light to come into the liquid crystal are roughly divided into two kinds, one of which is direct view type or transmissive type methods to let liquid crystal to transmit light emitted from the backlight provided in the backside, and the other is reflective type methods to reflect transmitted light incoming from the side of the viewer at the back of the liquid crystal and let the liquid crystal to transmit the reflected light to the viewer side. The development of reflective type liquid crystal displays has been vigorously proceeded, because this type of displays are power-saving, thin and lightweight without having to provide a backlight. In addition to the above advantages in terms of functions, the cost may be reduced, because members for the backlight are not necessary.
A number of conventional constructions have been proposed for such a reflective type liquid crystal display. (For example, various reflective type liquid crystal displays are described in LCD Intelligence: April 1997, pp. 54-58.) In the reflective type liquid crystal display, reflected light should have a scattering property in order to secure good visibility by the viewer, in other words, in order to permit light from all the parts of the picture plane to sufficiently reach the viewer. Therefore, the reflective type liquid crystal display must include an optical member for providing any of the components with a scattering characteristic. The constitutions of reflective type liquid crystal displays may be classified into the following three kinds in view of where such scattering characteristic is provided in the device: (1) such scattering characteristic is provided at the rearward of liquid crystal as viewed from the viewer, (2) the scattering characteristic is provided in the front of liquid crystal, and (3) the scattering characteristic is provided to the liquid crystal itself.
FIGS. 10A and 10B are views showing an example of a reflective type liquid crystal display in the type of (1). FIG. 10A shows an off state where a voltage is not applied to liquid crystal, and FIG. 10B shows an on state.
In FIGS. 10A and 10B, the viewer is in the upper part, in other words in the front of the liquid crystal, and light comes in from the upper part.
A polarizer 1 absorbs a polarized light component 101 parallel to the surface of the sheet and transmits a polarized light component 102 orthogonal to the surface of the sheet. Liquid crystal 3 is general TN (Twisted Nematic) liquid crystal having a twist angle of 90.degree.. The direction of the absorbing axis of a polarizer 5 is orthogonal (cross Nicol) to that of polarizer 1. Liquid crystal 3 is held between and in contact with orientation layers 9 and 9' for aligning the orientations of liquid crystal molecules. Transparent electrodes 10 and 10' are provided in the front and backsides of the liquid crystal/orientation layers, respectively, and apply a voltage to the liquid crystal. Transparent substrates 2 and 4 composed of glass, plastic or the like are provided in the front side and backside of the transparent electrodes, respectively. Provided in the backside of polarizer 5 is a reflecting board 6 having a surface of scattering type white resin or a metallic surface with a high reflectance such as silver and aluminum treated to have a scattering property.
Polarized light component 102 orthogonal to the surface of the sheet comes into liquid crystal 3, and has its plane of polarization twisted by 90.degree. by the TN liquid crystal having a twist angle of 90.degree. in the off state where a voltage is not applied between transparent electrodes 10 and 10'. Therefore, polarized light component 102 is converted into polarized light component 101 parallel to the surface of the sheet and transmitted through polarizer 5. Subsequently, polarized light component 101 parallel to the surface of the sheet, scattered by reflecting board 6 is once again transmitted through polarizer 5, converted into polarized light component 102 orthogonal to the surface of the sheet by liquid crystal 3, transmitted through polarizer 1, the light reaches the viewer and white display is made.
Meanwhile, in the on state where a voltage is applied between transparent electrodes 10 and 10', the twist of TN liquid crystal 3 is untwined, polarized light component 102 incident to the liquid crystal is transmitted through liquid crystal 3 without changing the direction of polarization and absorbed by polarizer 5. As a result, the light does not reach the viewer and black display is made.
At this time, if a color filter is inserted between the viewer and the reflecting board, color display is enabled.
If the directions of the absorbing axes of polarizers 1 and 5 are parallel rather than orthogonal, black display is achieved when the application of a voltage is off and white display is achieved when the application of a voltage to the liquid crystal is on, respectively. Color display is enabled in combination with the color filter.
FIGS. 11A and 11B show conventional reflective type liquid crystal displays in the type of (1), using a liquid crystal mode different from the above. In FIGS. 11A and 11B, the viewer is present in the upper part, in other words, in the front of liquid crystal, and incident light comes in from the upper part. Polarizer 1 transmits a polarized light component 101 parallel to the surface of the sheet and absorbs a polarized light component 102 orthogonal to the surface of the sheet. Liquid crystal 7 is guest-host type liquid crystal formed by adding a dichroic dye to nematic liquid crystal having positive dielectric anisotropy, orientation layers 9 and 9' are provided at the interface between the upper and lower substrates, and the liquid crystal is oriented in the direction parallel to the surface of the sheet.
Polarized light component 101 parallel to the surface of the sheet is transmitted through polarizer 1, comes into guest-host type liquid crystal 7 and is absorbed by the dichroic dye added to the liquid crystal when a voltage is not applied to guest-host type liquid crystal 7. As a result, the light does not reach the viewer, and black display is made.
Meanwhile, when a voltage is applied to guest-host type liquid crystal 7, liquid crystal molecules in guest-host type liquid crystal 7 are oriented in the direction of the electric field, in other words in the direction almost orthogonal to the substrate, while the added dichroic dye is also oriented in the same direction as the liquid crystal molecules, incoming polarized light component 101 is transmitted through guest-host type liquid crystal 7, scattered by reflecting board 6, once again transmitted through guest-host type liquid crystal 7 and polarizer 1 to reach the viewer, and therefore white display is made. Also as described above, by inserting a color filter between the viewer and the reflecting board, color display is enabled.
In the above conventional examples, a polarizer is necessary to turn on and off light transmitted through liquid crystal to display in white and black. When a polarizer is used in the reflective type liquid crystal display as described above, light is transmitted through the polarizer four times or twice in total before reaching the viewer. An ideal polarizer would not absorb light, but in practice about several to 10% light is absorbed and therefore the number of transmission of light through the polarizer is preferably as small as possible in order to provide bright display.
Furthermore, a conventional reflective type liquid crystal display cannot be used in a dark place, because there is little incident light. In order to solve this disadvantage, such a reflective type liquid crystal display is proposed in which a half mirror is used for the reflecting board or an opening is provided at a part of the reflecting board to transmit a part of light and a backlight is provided in the backside to use the display also as a transmissive type device. However, the reflecting board of such a structure has a low reflecting performance, the picture plane is disadvantageously darkened.